With the development of sophisticated mobile and industrial hydraulic systems, there is an increasing need for multi-purpose hydraulic fluids to satisfy the stringent requirements of these systems. Since the requirements of hydraulic fluids will vary, depending primarily on the type of pumps used, i.e., gear, piston or vane, fluids must be specifically designed to meet the performance requirements (power transmission, lubrication and cooling) of the particular system and obtain maximum pump life. The characteristics and utility of hydraulic fluids are determined by the base oil used and the particular combination of additives employed for the formulation thereof. Additives are selected to minimize viscosity changes, inhibit corrosion, increase thermal and oxidative stability, reduce wear, etc.
The viscosity of the fluid over the entire range of system operating temperatures is of primary concern for all hydraulic systems. There is a particular problem with mobile hydraulic equipment used in northern regions where nighttime temperatures often reach -40.degree. F. or below. The viscosity of petroleum-based hydraulic fluids in such equipment can exceed the maximum viscosity recommended by the manufacturer for safe startup. This is particularly so in the case of hydraulic equipment which utilizes vane pumps since the maximum recommended cold start viscosity for vane pumps is generally much lower than forpiston or gear pumps. For example, in Denison Bulletin 2002-C the maximum viscosity at cold start recommended for Denison vane equipment is 4000 SUS (862 centistokes) whereas for their piston equipment, the recommended maximum viscosity at cold start is 7500 SUS (1618 centistokes). Thus, following the above guidelines using SUNVIS 816 WR(32), a commercially available petroleum based hydraulic fluid manufactured by Sun Oil Company and approved by Denison as meeting their stringent HF-O standards, it would be possible to effect safe startup of piston pumps only at temperatures above about 5.degree. F. and vane pumps should only be started at temperatures of about 15.degree. F. or above. The hydraulic fluids should also maintain the initial viscosity characteristics under actual operating conditions and should not breakdown under stress and shear forces.
It would be highly advantageous, therefore, if hydraulic fluids were available which had improved low temperature viscosities permitting the safe startup of hydraulic equipment, particularly vane equipment, at lower temperatures than was heretofore possible. It would be even more advantageous if such fluids were based on readily available synthetic hydrocarbon oils and if these products had good shear stability and substantially met the Denison HF-O standards in regard to thermal stability, oxidative stability, hydrolytic stability and resistance to sludge formation and metal corrosion.